1. Field of the Invention
This invention relates to an optical disk drive, and is particularly concerned with a method for counting the number of tracks an optical head crosses during a seek operation in an optical disk drive.
2. Description of the Related Art
In an optical disk drive seek operation in which a laser beam (or an optical head) moves from a current position to a target position (or a target track) over the tracks, it is necessary to count the number of tracks the laser beam crosses during the seek operation, based on a TES (Tracking Error Signal), to obtain positional information of the moving optical head.
Optical structures to generate a TES are grouped into several classes, such as a push-pull method, a three-beam method, etc. However, a TES in any method, is a signal of a waveform showing that a positive peak and a negative peak occur alternately each time a laser beam crosses a land part 110 and a groove part 120 of an optical disk 100, as shown in FIG. 12A. Further, the TES is at a zero level when a laser beam passes through the land center 110A of the land part 110 and the groove center 120A of the groove part 120. FIG. 13 shows an example of conventional circuit structure to count the number of tracks an optical head crosses, based on a TES described above. In the figure, a window comparator 131, as shown in FIG. 14, generates a zero cross pulse if a TES is within a predetermined range of the zero level and a track count logic 133 counts the number of zero cross pulses.
On the recording surface of the optical disk 100, the positional information of a data recording part is usually written immediately before the data recording part in a pre-pitted form. Such positional information is called pre-pitted positional information and a pre-pitted portion thus obtained or a defect portion on the optical disk 100 causes noise within the TES when a laser beam crosses the pre-pitted portion or the defect portion. When such noise is caused, the above conventional circuit structure is apt to err in counting the number of tracks.
In the following, such a disadvantage in the circuit structure of the prior art is described in detail.
FIG. 15 shows the trajectory of a laser beam during a low-velocity seek operation. In the figure, a defect portion 150 usually spans more than one track on the recording surface of the optical disk 100. Accordingly, if the seek velocity is low, a laser beam passes through the defect portion 150 while the laser beam passes through one land part 110 and one groove part 120. This causes significant noise within the TES, as shown in FIG. 16. Said noise thus generated causes a pulse and frequently count errors in counting the number of tracks by the track count logic 133. The same problem is also caused if a laser beam passes through a pre-pitted portion. Further, other noises are apt to cause counting errors.